This invention relates to a method for preparing UV grade synthetic pyridine from the product mixture resulting from the conventional catalytic condensation reaction of aldehydes and/or ketones and ammonia or amines.
A wide variety of methods has been applied to the problem of purifying pyridine for its various uses. Thus, for example, Perrin et al., Purification of Laboratory Chemicals, Pergammon Press (1966), 248-249, describe conventional methods for separating pyridine from homologs. These include crystallization of the oxalates, complex formation with ZnCl.sub.2 or HgCl.sub.2, fractional crystallization, vapor phase chromatography, etc. Riddick et al., in Organic Solvents, 4th Ed., "Techniques of Chemistry", Vol. 2, Ed. A. Weissberger, J. Wiley & Sons, 1986, describe the purification of pyridine and mentioned some of the same techniques for separating pyridine from its homologs. Also mentioned is treatment with cerium sulfate, distillation from potassium hydroxide or barium oxide, treatment with alumina, chromatography, fractional distillation (to prepare pyridine suitable for use in chemical reaction studies), etc.
It is further known that fractional distillation and azeotropic distillation techniques have been utilized in separations involving other aromatic bases for various purposes. Thus, U.S. Pat. No. 3,493,473 removes a variety of impurities from pyridine by azeotropic distillation of impurity-water azeotropes. Impurities include benzene, toluene, xylenes, nitriles, permanganate reducible materials and Ehrlich's reagent positive materials. U.S. Pat. No. 3,431,266 distills from a mixture of pyridine sulfate and sulfuric acid, a pyridine/water mixture exceeding the azeotrope. Subsequently, a pyridine/water azeotrope is freed from the distillate. However, "pyridine" is defined to include homologs such as the picolines, glutidines, quinolines, etc. U.S. Pat. No. 2,363,158 uses a pyridine/water hydrocarbon azeotrope to separate pyridine from piperidine. U.S. Pat. No. 2,058,435, on the other hand, discloses how the pyridine/water azeotrope can disturb efforts to purify pyridine (col. 1, lines 20-25). U.S. Pat. No. 2,425,220 applies azeotropic distillation to the recovery of pyrrole from fractions of heterocyclic nitrogen compounds including picolines and lutidines using a pyrrole/water azeotrope. U.S. Pat. No. 2,335,823 separates 2,6-lutidine, 3-picoline and 4-picoline from a mixture by sequentially distilling the water azeotropes of each of these.
However, none of these references deals with the problem of purifying pyridine to the extent necessary to meet commercial UV grade specifications. The pyridine-containing reaction mixtures which result from conventional synthetic methods for preparing pyridine do not meet UV specifications. But the latter must be met for a variety of commercial uses of pyridine, e.g., as a specialty solvent.
Pyridine is prepared industrially by catalytic condensation of aldehydes and/or ketones with ammonia and/or amines. A wide variety of catalysts, reactants, feed ratios, reaction conditions, is known. See, e.g., U.S. Pat. Nos. 4,675,410, 4,149,002, 3,907,915, 3,946,020 and 2,807,618; "Heterogeneous Conversion of Acylic Compounds to Pyridine Bases--A Review", Applied Catalysis, 23, (1986), 1-14; "Pyridine and Pyridine Derivatives", Goe, Gerald L., Kirk-Othmer, 3rd Edition, Vol. 19, John Wiley & Sons, p. 454 (1978); "Synthetic and Natural sources of the Pyridine Ring", Bailey et al., pp. 1-252 in "Heterocyclic Compounds", Volume 14, "Pyridine and Its Derivatives", John Wiley & Sons, New York (1984); and many other references, all of which are entirely incorporated by reference herein. The literature is devoid of any mention of how UV grade pyridine can be recovered from the product of such reactions.
Although it is known that the reaction mixtures fail to meet UV specifications, it is not known which components of such mixtures are responsible for the undesired UV absorption. Because of the high complexity of the impurity spectrum of these mixtures, determination of their compositional details is a difficult problem.
In attempting to remove the unknown UV impurities, many of the previously successful chemical purification methods for pyridine were applied. In all cases, they failed to produce UV grade pyridine. Similarly, conventional fractional distillation also failed to produce UV grade pyridine.